Diesel engines produce an exhaust gas that contains a variety of pollutants that are the subject of environmental legislation around the world. These pollutants include carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). To meet permissible levels of pollutants that may be emitted into the atmosphere set by legislation, exhaust systems for treating the exhaust gas produced by a diesel engine generally contain several emissions control devices. In such exhaust systems, the exhaust gas is usually passed to a first emissions control device that is able to oxidise carbon monoxide (CO) and the unburned hydrocarbons (HCs) that are present in the gas.
In addition to the pollutants mentioned above, the exhaust gas may contain impurities that originate from the diesel engine lube oil and/or the specific type of fuel that has been used. These impurities typically include an element from the p-block of the periodic table, particularly from the third row (period 3) of the periodic table, such as phosphorus and sulfur. Some of these impurities may also form pollutants that are the subject of environmental legislation.
The first emissions control device in an exhaust system to encounter the exhaust gas produced by a diesel engine, particularly for heavy duty applications (e.g. trucks, buses, off-road vehicles, trains, tractors, ships), is typically a diesel oxidation catalyst (DOC). The DOCs used in heavy duty applications are generally subjected to prolonged periods of continuous use during each journey or drive cycle and are expected to have a relatively long lifetime. Any impurities in the exhaust gas are often deposited in the DOC (the first emissions control device encountered by the exhaust gas) and accumulate over its lifetime. These impurities degrade the performance of the DOC because they can poison catalytically active species, permanently occupy sites on the surface of the catalyst or chemically alter the catalyst composition of the DOC. As a result of this degradation in performance, it may be necessary to replace a DOC after it has been used in a vehicle that has travelled for several hundreds of thousands of kilometers.